In the following description the term “turbine” is used to refer to rotary engines having a stator and a rotating part force coupled by a fluid medium such as water or gas. Of particular interest for the present invention are axial turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades. Movements are generally registered as movements relative to a casing or housing.
A common problem encountered in the design and operation of turbines is the leakage between the rotor shaft of a stationary steam turbine and static casing or extensions thereof.
To reduce leakage it is known to close the gap between the rotating parts and the static parts by appropriate seals. The sealing devices used in turbines are known as steam seals, gland seals, labyrinth seals, steam packing, diaphragm packing, and pressure packing. The fundamental component of these seals is the sealing ring. The sealing ring is a ring shaped structure which is typically provided as arcuate sealing ring segments. The sealing ring encircles the rotating member and bridges or blocks at least partly the gap between the rotating part and the surrounding stationary part.
The most common type of seal used for this purpose is the labyrinth seal. A labyrinth seal has typically a number of radially extending annular knifes, fins or teeth on one part and a corresponding annular seal land on the other part or an arrangement of threats or grooves. More complex seals include radially extending, interlocking annular knifes, fins or teeth on both sides of the gap. All variants have the common feature of providing a tortuous path for the fluid through the gap. As a complete ring, a labyrinth seal is usually assembled as halves, quarter or smaller segments within and supported by the casing.
As labyrinth seals are well known, it suffices for the purpose of the present invention to emphasize that such seals are complex shapes requiring exacting dimension tolerances to function properly. Any movement of the parts of the seal from their default positions or wear during operation generates usually a significantly increase of leakage or friction between the moving and the static part.
To accommodate relative movement of the parts of the seal in case of a radial expansion or shrinkage of the blade, some seals are assembled as spring-backed packages. In a spring-backed seal, the elastic force pushes one part of the seal against the other and thus avoids widening gaps or excessive friction when the moving blades shrink or expand.
Labyrinth-type seals are described in a large body of published literature, exemplarily represented by the U.S. Pat. Nos. 4,420,161, 5,735,667, or 6,131,910, respectively, all of which can be used to derive further background knowledge for the present invention.
While improving the performance of labyrinth seals is a perpetual task, the current invention addresses further problems specifically associated with the retrofitting of existing stationary turbines, particularly steam turbines.